MELBOURNE, Australia -- An abdominal hernia has forced UFC middleweight champion Robert Whittaker out of his highly anticipated title defense against Kelvin Gastelum at UFC 234, just hours before doors were scheduled to open at Rod Laver Arena.

UFC president Dana White told ESPN that Whittaker complained of pain in his abdomen late last night, and was taken to a local hospital. The Aussie champ was diagnosed with a hernia and will undergo surgery later in the day, according to White.

A non-title middleweight bout between rising star Israel Adesanya (15-0) and Anderson Silva (34-8) will now headline UFC 234, which sold out in minutes when tickets first went on sale. Adesanya, who fights out of New Zealand, is one of the UFC's most promising prospects, taking on an all-time great in Silva.

It's obviously a huge blow to Whittaker, 28, who was forced to pull out of a title defense on home soil in February 2018. That withdrawal was also due to injury. Representatives for Whittaker did not immediately respond to requests for comment.

Silva, 43, broke into tears during the UFC 234 official weigh-in. This is Silva's first appearance since February 2017. The UFC had already announced the winner of that fight would be next in line to challenge for the 185-pound title.